1. Field of the Invention
The present invention relates to a process for vapor phase epitaxy of a compound semiconductor, and particularly to a process for growing indium gallium nitride (In.sub.x Ga.sub.1-x N, where, 0&lt;x&lt;1) by vapor phase epitaxy.
2. Description of Related Art
In FIG. 1, there is shown a diagrammatic sectional view of a structure of a blue or green light emitting diode (LED) element of a type of gallium nitride (GaN) on a sapphire substrate, and such a structure is presently put on the market and described, for example, in "Nikkei Science" October 1994, page 44.
This blue or green LED element includes an epitaxial wafer comprising a sapphire substrate 11, a GaN buffer layer formed on the substrate 11, and a hexagonal GaN epitaxial layer 13 formed on the GaN buffer layer 12. The LED element further includes a clad layer 14, a light emitting layer 15, a clad layer 16 and GaN epitaxial layer 17 formed, in turn, on the wafer, and is also provided with ohmic electrodes 18 and 19. Furthermore, the GaN buffer layer 12 in the LED element is placed for decreasing defects by the differences of lattice constants between the sapphire substrate 11 and the GaN epitaxial layer 12.
In the above blue or green LED element, the substrate 11 is made of sapphire with an insulation property. When the electrodes 18 and 19 are formed on the element, it is also necessary for these electrodes to be formed on the same face side of the element. Therefore, forming the electrodes requires complex processes, for example, at least two patterning processes by photolithography and further an etching process by reactive ion etching.
Further, because hardness of sapphire is large, there is a problem that the sapphire substrates is difficult to be cut when separating into the individual elements. In an aspect of application of the LED element, furthermore, because sapphire cannot be cleaved, the element cannot be used as a laser diode which acts as an optical resonator with cleaved surfaces.
Thus, instead of sapphire with these drawbacks, conductive gallium arsenide (GaAs) is tried to be used as a substrate. Namely, it is studied for gallium nitride (GaN) to grow on a GaAs substrate by metal-organic chloride vapor phase epitaxy techniques, so-called "MOCVPE". This growth of GaN is sufficiently faster than growths by prior organometallic vapor phase epitaxy techniques, so-called "OMVPE".
Methods by MOCVPE techniques use, as raw materials, chlorides of elements in the third group of the element periodic table, and allow GaN to grow rapidly. The methods also allow growth of indium gallium nitride (InGaN) which provides an active layer of an LED element.
Further, it is desired to realize a pure blue wavelength LED element with high indium (In) mole fraction "x" of In.sub.x Ga.sub.1-x N. However, as this In mole fraction "x" increases in In.sub.x Ga.sub.1-x N, it is necessary to decrease a growth temperature on epitaxy. As a result, decreasing the temperature causes a problem that a growth rate of the epitaxy is slower.
Additionally, in "Kristall und Technik" Vol.12 No.6 (1977), pages 541 to 545, it is reported that chloride, namely indium trichloride (INCl.sub.3), is used as an In source and ammonia (NH.sub.3) is used as a nitride source, so that a hexagonal indium nitride (InN) crystal is grown on a sapphire substrate. However, the InN growth on a GaAs substrate is not examined Accordingly, cubic InN and InGaN adapted to manufacturing a laser diode are not obtained.
In the prior MOCVPE techniques that aluminium (Al), gallium (Ga) and indium (In) of elements in the third group of the element periodic table are provided as chlorides, a metal-organic raw material including one of the elements in the third group, for example, trimethylindium (TMIn: C.sub.3 H.sub.9 In) and trimethylgallium (TMGa: C.sub.3 H.sub.9 Ga), is provided together with hydrogen chloride (HCl). In the case of the former example, TMIn and HCl are composed into indium chloride (InCl) and the InCl is reacted with ammonia (NH.sub.3) gas to grow InN and InGaN grown on a substrate, for example, a GaAs substrate.
However, these MOCVPE techniques lead to a slow growth rate on the substrate and cannot reproduce a certain composition of InGaN.
On the other hand, when vapor phase epitaxy (VPE) of compound semiconductor is performed, selection of atmosphere and carrier gasses is one of important key factors that strongly affect quality of the compound semiconductor to be formed.
For example, Japanese patent Laid-open Publication No.Showa (J-PA) 49-121478 (Hitachi) discloses a VPE technique that compound semiconductor is grown by vapor phase epitaxy with an inactive gas. Hydrogen (H.sub.2) gas used before this technique has smaller specific gravity than raw material gasses and doping material gasses. The technique of Hitachi selects argon (Ar) gas which has a specific gravity close to raw material and doping material gasses, so that these raw material and doping material gasses are mixed uniformly to obtain uniformity of epitaxial layer and homogeneous impurity concentration.
Further, in Japanese patent Laid-open Publication No.Showa (JP-A) 51-126036 (Fujitsu), there is described another VPE process for growing a semiconductor crystal, which provides a carrier gas including an inactive gas with adjunctive hydrogen (H.sub.2) gas less than 0.02 at volume ratio to the inactive gas. If only H.sub.2 gas is used as a carrier gas, the H.sub.2 gas reduces quartz (SiO.sub.2) forming a wall of a reaction furnace to produce impurities of silicon (Si), and inhomogeneity of reacting gas concentration is caused chiefly by the difference of molecular weights between a raw material gas and the H.sub.2 carrier, because the furnace has a structure of a horizontal type. In this process of Fujitsu, inactive gas, such as nitrogen (N.sub.2) gas, argon (Ar) gas, etc., is used as a carrier gas for sending arsenic trichloride (AsCl.sub.3) into the furnace, so that the inactive gas does not happen to react with the material of the wall at all, and an epitaxial layer of the crystal is allowed to grow rapidly and to be homogeneous.
In the process (Fujitsu), further, since the H.sub.2 gas of a very small quantity is added adjunctively to the inactive carrier gas, crystal growth and impurity concentration are well controlled and maintained at certain levels. Namely, without the H.sub.2 gas, AsCl.sub.3 repeats thermal decomposition and recombination, according to the following reversible reactive equation (1), and as a result the crystal growth becomes remarkably unstable. But, with a very small mount of the adjunctive H.sub.2 gas, an effective reaction, according to the further following reversible equation (2), and this reaction saturates at the volume ratio 0.02 of H.sub.2 gas to the inactive gas: ##EQU1##
Furthermore, in Japanese patent Laid-open Publication No.Showa (JP-A) 58-167766 (Kogyogijutsu-in), there is described a VPE apparatus in which a raw material gas is introduced upwards from the bottom and discharged out of the top. If a raw material gas has heavier molecular weight than a carrier gas, there are problems that the raw material gas is distributed inhomogeneously in the carrier gas and thickness of a layer is also distributed inhomogeneously. In this apparatus of Kogyogijutsu-in, because a carrier gas such as nitrogen (N.sub.2) gas, argon (Ar) gas, etc., is introduced upwards from the bottom, the inhomogeneous distribution of deposited layer on a substrate is improved.